The hydraulic actuation of the clutch pedal leads to sputtering phenomena of the engine when abruptly removing the foot from the clutch pedal, especially when operating the vehicle in the lower gears of the engine. To solve this problem, German Publication DE 1785 882 discloses an actuating device for vehicle clutches, which starts from an approach in which the slave cylinder is connected to an upstream non-return valve whose valve stem is provided with a return-flow hole for the hydraulic fluid in order that, when the clutch pedal is abruptly released, the return movement of the piston located in the slave cylinder is slowed down as a result of the throttling by the return flow movement. However, the deceleration cannot be achieved very effectively by throttle bores of this kind since the throttle bores must be kept very small in diameter because of the strong clutch spring forces. For that reason, German Publication DE 1 785 882 describes a solution where in the slave cylinder, a restricting plate is disposed between the cylinder bottom having the inlet bore and the slave piston, the restricting plate being acted upon by an energy storage device in a direction from the slave piston to the cylinder bottom. The restricting plate has a plurality of radially extending throttling grooves for decelerating the fluid flow so as to avoid sputtering phenomena of the engine in response to sudden movements of the clutch pedal.
In German Publication DE 37 36 584 A1, the objective of designing a hydraulic actuator such that peak loads are avoided even during extreme driving, which is associated with extreme clutch engagement speeds, and thus, with torque peaks, is attained by providing a throttle valve in the hydraulic line between the master cylinder and the slave cylinder. In this manner, the fluid flow is throttled.
Moreover, German Patent DE 43 34 551 C2 discloses a further hydraulic actuator for a clutch in the drivetrain of a motor vehicle. The objective achieved in this document is to provide, between the master cylinder and the slave cylinder in the hydraulic line connected to the actuator, a valve body in the form of a throttle valve whose closure member can be moved in a reliable way when lifting it off the sealing surface. This objective is achieved in that the closure member is provided with a cup-shaped extension in which is carried the compression spring, and in that the outer contour of the closure member facing the inner wall of the valve housing is designed such that hydraulic fluid may pass through in-between in an unhindered manner.
With these prior art solutions for an additional device disposed in the hydraulic path for actuating a vehicle clutch, the engagement speed of the clutch is reduced when the clutch is engaged with extremely rapid return movements of the pedal. This is done by throttling the fluid flow toward the master cylinder, thereby reducing the clutch engagement speed. The proposed solutions for throttling may possibly reduce rapid return movements. However, during normal operation, additional pressure reductions occur, which are perceived by the driver as an unpleasant decrease in pedal force. Moreover, this kind of throttling results in that the pressure reductions produced by these devices are temperature-dependent. This may lead to unwanted, greater pressure reductions, especially at low temperatures.